1. Field of Industrial Use
This invention relates to a magnetic noise reducing device for reducing the magnetic noise level and improving the S/N ratio in a setting for measuring the biomagnetism of a living body having a very low magnetic level in order to improve the reliability of the outcome of the measurement.
2. Prior Art
Reduction of the environmental magnetic noise level and improvement of S/N ratio is a serious concern in the measurement of the biomagnetism of a living body using a SQUID flux meter.
As illustrated in FIG. 2(A) of the accompanying drawings, the environmental magnetic noise is correlated with 1/f (f=frequency) in the low frequency range, showing a level between 1.times.10.sup.-10 and 1.times.10.sup.-9 T(Hz).sup.-1/2 for the frequency of 1 Hz and a higher level for lower frequencies. On the other hand, the level of the biomagnetism of a living body to be detected is found between 10.sup.-14 and 10.sup.-9 (depending on the intensity, the depth, the direction of emission of the signal generating source).
The frequency bandwidth (B) involved in the measurement of biomagnetism is normally around DC to 1 kHz. Since the noise level in a setting for measuring the biomagnetism of a living body is expressed by Bnx B.sup.1/2 (where Bn is magnetic noise per unit frequency), it will be approximately thirty two (32) times as large as that of 1.times.10.sup.-9 T(Hz).sup.-1/2 for DC to 1 kHz. Besides, the frequency characteristics of the noise involved need to be taken into consideration. If the noise level is of a magnitude of 5.times.10.sup.-9 T and a magnetic attenuation in the order of 10.sup.-14 is involved, the S/N ratio will be 1 for a signal of 10.sup.-14.
In view of the above facts, a magnetic noise reducing device is required to have a magnetism damping factor of 10.sup.5 to 10.sup.6 and be specifically effective for low frequency noises (less than 1 Hz) if it is effectively used for measuring biomagnetism.
Known means for reducing magnetic noise and means for canceling environmental magnetic noise include the followings.
a) magnetic shield room
This is a room constructed by using magnetically highly permeable materials such as Permalloy and providing magnetic noise damping areas inside the room.
FIG. 5 is a graph showing the frequency dependency of the magnetism damping factor of such a magnetic shield room. In FIG. 5, curves L.sub.1 and L.sub.2 show the magnetic damping factors of two magnetic shield rooms prepared by using walls having a multi-layered structure of aluminum and Permalloy plates. The magnetic damping factor of such a room is increased as the number of aluminum and Permalloy layers grows.
As is obvious from FIG. 5, such a magnetic shield room does not show a large magnetism damping factor for low frequency magnetic noise. Moreover, such a room inevitably provide only a narrow closed space for measurement and entails a high building cost of several hundred million yen.
b) electric method for canceling magnetic noise
This is a method using a device as illustrated in FIG. 6 to eliminate the noise component of output signal by determining the difference between the measurement of a signal detecting SQUID flux meter SC1 and that of a reference SQUID flux meter SC2. In FIG. 6, arrow M denotes signal, arrow N denotes noise, P1 and P2 denote controllers, Q and R respectively denote a signal processing circuit and the output terminal of the circuit.
Since such a device does not and cannot remove the magnetic field existing in the signal detecting space, corrective means may be additionally required for each of the channels involved if a multi-channel SQUID system is used.
c) gradiometer
As is known, a SQUID flux meter normally comprises a magnetic flux transformer constituted by a magnetic flux detecting coil MC, an input coil IP and a SQUID inductance SI as illustrated in FIG. 7. If the spatial gradient is known for the magnetic field to be detected, the gradiometer to be used for magnetic noise reduction are so designed that any magnetic fields having a gradient lower than it may be canceled.
If a magnet meter U, a primary differential gradiometer U1 and a secondary differential gradiometer U2 are arranged as illustrated in FIG. 8 to form a gradiometer for the purpose of magnetic noise reduction, such a gradiometer can selectively cancel even magnetic fields (primary differential) and/or magnetic fields up to the primary gradient (secondary differential) on site or produce .delta.Bz/.delta.x, .delta.Bz/.delta.y and/or .delta.Bx/.delta.z according to the purpose of measurement.
A gradiometer as described above is, however, accompanied by the disadvantage of a low sensitivity of a SQUID flux meter for magnetic flux density of higher orders, the sensitivity being remarkably lowered in terms of distance (in the Z direction for .delta.Bz/.delta.z) so that the canceling efficiency is determined by the winding balance, making the rate of noise reduction normally as poor as 10.sup.2 to 10.sup.4.
Contrary to the above described disadvantages of known magnetic noise reducing devices, a magnetic noise reducing device according to the invention is based on the principle of nil magnetic field detection that has been used for SQUID flux meters comprising a SQUID circuit having a feedback loop for making a SQUID element always show a nil magnetic field intensity. In other words, in a magnetic noise reducing device according to the invention, the feedback current of each SQUID flux meter it comprises is supplied not to a SQUID element but to corresponding noise canceling coils, the magnetic flux detecting coil of the SQUID flux meter being arranged within a given space defined by the surrounding noise canceling coils in order to operate the said magnetic flux detecting coil as a zero-level detecting coil so that said space defined by the noise canceling coils provides a space that can be effectively shut out magnetic noise.
More specifically, according to the present invention, there is provided a magnetic noise reducing device comprising an appropriate number of noise canceling coils and corresponding matching SQUID flux meters each having a magnetic flux detecting coil arranged within a specific space defined by the corresponding noise canceling coils, the output of each of said SQUID flux meter being supplied to said corresponding noise canceling coils as a feedback current.
Thus, with a magnetic noise reducing device according to the invention, a feedback current is supplied to a number of noise canceling coils not from an ordinary feedback circuit but from the corresponding SQUID flux meter.
Since the magnetic flux detecting coil of each of the SQUID flux meters is arranged within a magnetic space defined by the corresponding noise canceling coils, it operates as a nil detector. Thus, the noise canceling coils can provide a zero-magnetism space within a given space regarding the environmental magnetic noise.
Now, the present invention will be described in greater detail by referring to the accompanying drawings that illustrate preferred embodiments of the invention.